The oldest rocks in the area are the igneous and metamorphic rocks which form the Malvern Ridge. These were formed during the Precambrian period and are approximately 677 million years old. During Precambrian times the shape and position of the continents were completely different from today. England and Scotland were separated by an ocean which is referred to as the Iapetus Ocean. Scotland was joined with North America and Greenland to form the northern hemisphere Laurentian continent. England was located on the southern shore of the southern hemisphere continent of Gondwana, which was located approximately 60 Degrees south of the equator (Figure Two).
The Precambrian rocks exposed in the Malvern Hills have been subdivided into two separate units. These are referred to as the Malverns Complex and the Warren House Formation. The Malverns Complex is comprised of a suite of coarse-grained igneous rocks which were formed within a magma chamber deep within the Earth’s crust. These intrusive igneous rocks mainly consist of diorites, tonalites and granites. This rock association is referred to as a calc-alkaline suite which is normally associated with the subduction zones of volcanic island arcs (see below).
Volcanic Island Arcs
Volcanic island arcs are found at the edges of shrinking oceans. During Precambrian times the tectonic processes operating in the Malvern area were thought to be similar to those creating the present day island arc systems in Japan.
Island arcs are formed by the collision between two oceanic plates. One plate is subducted (dragged) beneath the other along a trench called a subduction zone. At a depth of between 80 – 100 km partial melting occurs and the molten rock (magma) rises towards the surface to form a volcano. Some magma remains trapped within the Earth’s crust and cools slowly to form coarse-grained igneous rocks. The Malverns Complex largely consist of a series of intrusive igneous rocks called tonalite, diorite and granite.
The rocks exposed at Tank Quarry are Diorite and Granite (Figure Three). These are intrusive igneous rocks and are coarse-grained in texture. The grain size (or crystal size) of an igneous rock is influenced by the rate at which the magma cools and solidifies. Igneous rocks formed deep within the Earth’s crust are coarse-grained. Magma trapped for beneath the Earth’s surface cools slowly, forming large crystals, of several millimeters size. The rapid cooling of extrusive rocks gives fine-grained material with small crystals, usually less than a millimetre in size.
Although there is little doubt that the Malverns Complex is igneous in origin the texture of the rocks have been modified by tectonic forces to produce a distinct “metamorphic texture” normally associated with metamorphic rocks such as schist or gneiss (Figure Three). The rocks at the southern end of the hills demonstrate the greatest intensity of deformation. The Malverns complex has also been cut by a series of microdiorite and pink pegmatite dykes. It is likely that these dykes were formed by a later phase of igneous activity while the Malvern Complex was still cooling. These intrusions are more common at the northern end of the Hills (Photograph Two).
Warren House Formation
The rocks belonging to the Warren House Formation occupy approximately 1 km Sq. of the Hills and exposures can be found on Tinkers Hill, Broad Down and Hangman’s Hill. The Warren House Formation is comprised of extrusive igneous rocks formed by volcanic activity approximately 566 million years ago. The basalts exposed at Clutters Cave suggest that part of the Warren House Formation was formed by volcanic activity on the sea floor and are similar in composition to basalts associated with present day island arcs in the western Pacific (Photograph Three). However, the presence of lavas comprised of rhyolite indicates that surface volcanic activity was also occurring during this time and the pyroclastic rocks exposed at Reservoir Quarry show that these volcanic eruptions were extremely violent.